The present invention relates to a method and a device for actuating a member made of a shape memory alloy material, and in particular, the present invention relates to such an actuating method and device which utilize the Joule effect produced by passing an electric current through a shape memory alloy member for heating the member, the member being in the prestressed state and the parameters of the wave form of the electric current being particularly tailored so as to best actuate and utilize the shape recovering property of the shape memory alloy of which the member is made, in order to perform thermal/mechanical energy conversion.
There are various known forms of thermal/mechanical energy conversion devices utilizing the shape recovering property of shape memory alloy in order to perform thermal/mechanical energy conversion. Typically, these energy conversion devices are utilized in robot manipulators and the like. When a member made of shape memory alloy is supplied with an input of thermal energy and heated, as for example by the Joule effect which is produced by an electric current passed through the member, the member exerts a force in the direction which will bring its shape closer to the original shape via a phase transformation (the reversion transformation from the martensite phase to the parent phase), and its shape that tends to alter towards an original shape it remembers. This force tending towards the alteration of the shape of the member is utilized for driving a driven member in a particular desired direction, thus performing mechanical work.
This use of the Joule effect for heating a member made of shape memory alloy to actuate it is per se known. However, various problems tend to arise with this heating method. Particularly, the various parts of the member are not heated evenly, and large temperature differentials are produced in the member. This means that certain portions (at least) of the member are heated up far beyond the temperature region in wihch phase transformation takes place, thus causing heat damage to the alloy in these portions, and shortening of the operational life of the member.